The present invention generally relates to keyboard assemblies of the type that use a plastic key to compress a silicone rubber pad so that a conductive layer on the pad bridges a plurality of contact pads on a printed circuit board thereby completing an electrical circuit, and more particularly the invention is directed to improvements in such assemblies that make operation of the keyboard more reliable and provide a better "feel" to the operator.
Keyboard assemblies of the type to which the present application is directed are used in a variety of consumer electronic products and appliances such as television receivers, calculators, security system keyboards and the like. The invention is not, however, limited to any particular application and has utility in any application where improved reliability of operation and enhanced user "feel" are desired.
In recent years, the trend in the design of keyboards for user input to an electronic device or system has been to make the keyboard as thin as possible. There has evolved a keyboard assembly having a thin sandwich construction comprising a face plate through which individual keys project, a silicone rubber pad and a printed circuit board. The individual keys have an enlarged base which serves to retain the key within the face plate. This enlarged base engages a corresponding convex dimple formed in the silicone rubber pad that acts as the return spring for the key when it is pressed and then released. On the opposite side of the convex dimple is a conductive coating which, when the key is depressed, bridges a plurality of contact pads on the printed circuit board. One common design employs three contact pads in an offset alignment to allow for a close spacing of the pads. The center pad is connected to electrical ground, and in order to insure proper operation of any given key, the other two pads must be electrically connected to the center pad by operation of the key.
This keyboard assembly design produces a pleasing appearance which has found good commercial acceptance. However, by making the keyboard assembly very thin, very little lateral support is provided for the keys as they reciprocate within the apertures of the face plate. As a result, unless the user is careful in depressing a key, the key may be depressed at an angle to the printed circuit board. This can cause the silicone rubber pad to bridge only two of the three pads or perhaps only one of the three pads resulting in an erroneous keyboard input. Moreover, the poor wobbly feel of the keys is not desirable for attaining commercial acceptance.